Precision oscillators that use imprecise components

ABSTRACT

Trimming components within an oscillator comprising: a trim-capable current source, wherein the trim-capable current source comprises a trimmable resistor and a trimmable current component, a comparator comprising a first input terminal that couples to the trim-capable current source and the second input terminal that couples to a reference voltage source, a switch coupled to the first input terminal and the trim-capable current source, and a trim-capable capacitor coupled to the switch, wherein the switch is coupled between the trim-capable capacitor and the trim-capable current source.

BACKGROUND

Oscillators are electronic devices that produce periodic, outputsignals, such as sine waves, square waves, or triangle waves. Togenerate oscillating, output signals, oscillators often convert directcurrent (DC) received from a power source to an alternating current (AC)signal, By utilizing an oscillator's output signals, circuit designersare able to utilize oscillators for a variety of electronic systemsranging from clock generation in control logic components (e.g.,microprocessors), transmitting signals for transmitter devices,producing audio sounds, and performing carrier synthesis in cellulartechnology. Depending on the application, oscillators can exhibitdifferent topologies and performance parameters. As an example, circuitdesigners may employ low frequency oscillators (e.g., about 20 hertz(Hz)) in audio synthesizing applications while radio frequency (RF)oscillators produce output signals that range in frequencies of about100 kilohertz (kHz) to 100 gigahertz (GHz).

Circuit designers may prefer external oscillators instead of internaloscillators in certain applications requiring relatively high precisionand stable output signals. Internal oscillators, which are also known aszero-pin oscillators, are generally less precise because of the devices'susceptibility to noise and/or temperature variation. The imprecisiondrawbacks for internal oscillators can originate from fabricationtechnology and process variations. For instance, a manufacturer may usepolysilicon material to fabricate components, such as poly-resistorcomponents, within the internal oscillator. Unfortunately, due toproperties of the polysilicon material, polysilicon-based components canhave a relatively high temperature coefficient (e.g., about a 2,500parts per million (ppm) per degree Celsius (° C.) for a poly-resistorcomponent) that affects the component's attributes as operationtemperature changes. Although manufacturers may use other types ofcomponents with lower temperature coefficients, such as silicidepoly-resistor component (e.g., about 100-200 ppm/° C.) or zerotemperature coefficient of resistance (ZTCR) component (e.g., less than50 ppm/° C.), the cost of fabricating internal oscillators with the moreprecise components generally tend to be more expensive. Thus, being ableto improve the accuracy of internal oscillators without increasingfabrication costs remains valuable in fabricating precision oscillators.

SUMMARY

The following presents a simplified summary of the disclosed subjectmatter in order to provide a basic understanding of some aspects of thesubject matter disclosed herein. This summary is not an exhaustiveoverview of the technology disclosed herein. It is not intended toidentify key or critical elements of the invention or to delineate thescope of the invention. Its sole purpose is to present some concepts ina simplified form as a prelude to the more detailed description that isdiscussed later.

In one implementation, an oscillator including: a trim-capable currentsource, wherein the trim-capable current source includes a trimmableresistor and a trimmable current component; a comparator including afirst input terminal that couples to the trim-capable current source andthe second input terminal that couples to a reference voltage source, aswitch coupled to the first input terminal and the trim-capable currentsource; and a trim-capable capacitor coupled to the switch, wherein theswitch is coupled between the trim-capable capacitor and thetrim-capable current source.

In another implementation, an oscillator including: a trim-capablecurrent source configured to: trim one or more resistance values thataffects a first portion current of a capacitor charging currentgenerated from the trim-capable current source and trim the firstportion current to adjust a proportion of the first portion current inthe capacitor charging current generated from the trim-capable currentsource; a comparator including a first input terminal that couples tothe trim-capable current source and a second input terminal that couplesto a reference voltage source; and a trim-capable capacitor coupled tothe first input terminal, wherein the trim-capable capacitor isconfigured to perform capacitor switching that controls a charge time ofthe trim-capable capacitor.

In yet another implementation, a method including: trimming, for arelaxation oscillator, one or more resistance values that affects afirst portion current of a capacitor charging current generated from atrim-capable current source located within the relaxation oscillator,trimming, for the relaxation oscillator, the first portion current toadjust a proportion of the first portion current in the capacitorcharging current generated from the trim-capable current source, whereinthe capacitor charging current is a combination of the first portioncurrent and a second portion current; and trimming, for the relaxationoscillator, a trim-capable capacitor to perform capacitor switching thatcontrols a charge time of the trim-capable capacitor.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of various examples, reference will now bemade to the accompanying drawings in which:

FIG. 1 is a block diagram of an oscillator in accordance with variousimplementations.

FIG. 2 illustrates a waveform diagram that provide waveform graphs thatcorrespond to the operation of the oscillator shown in FIG. 1.

FIG. 3 is a schematic diagram of another oscillator in accordance withvarious implementations.

FIG. 4 illustrates a waveform diagram that includes waveform graphs thatcorrespond to the operation of the oscillator shown in FIG. 3.

FIG. 5 is a schematic diagram of an implementation of a trim-capablecurrent source component configured to perform a resistance value trimand temperature slope trim.

FIG. 6 is a schematic diagram of an implementation of a trim-capablecurrent source configured to perform a temperature slope trim to furthertune the capacitor charging current Ic.

FIG. 7 is a schematic diagram of another implementation of trim-capablecurrent source configured to perform a temperature slope trim to furthertune the capacitor charging current Ic.

FIG. 8 is a schematic diagram of an embodiment of trimmable capacitorcomponents.

FIG. 9 is a flow chart of an implementation of a method to trim multiplecomponents within a precision oscillator to stabilize and reach a targetfrequency.

While certain implementations will be described in connection with theillustrative implementations shown herein, the invention is not limitedto those implementations. On the contrary, all alternatives,modifications, and equivalents are included within the spirit and scopeof the invention as defined by the claims. In the drawing figures, whichare not to scale, the same reference numerals are used throughout thedescription and in the drawing figures for components and elementshaving the same structure, and primed reference numerals are used forcomponents and elements having a similar function and construction tothose components and elements having the same unprimed referencenumerals.

DETAILED DESCRIPTION

Certain terms have been used throughout this description and claims torefer to particular system components. As one skilled in the art willappreciate, different parties may refer to a component by differentnames. This document does not intend to distinguish between componentsthat differ in name but not function. In this disclosure and claims, theterms “including” and “comprising” are used in an open-ended fashion,and thus should be interpreted to mean “including, but not limited to .. . .” Also, the term “couple” or “couples” is intended to mean eitheran indirect or direct wired or wireless connection. Thus, if a firstdevice couples to a second device, that connection may be through adirect connection or through an indirect connection via other devicesand connections. The recitation “based on” is intended to mean “based atleast in part on.” Therefore, if X is based on Y, X may be a function ofY and any number of other factors. The terms “a,” “an,” and “the” arenot intended to refer to a singular entity unless explicitly so defined,but include the general class of which a specific example may be usedfor illustration. The use of the terms “a” or “an” may therefore meanany number that is at least one, including “one,” “one or more,” “atleast one,” and “one or more than one.” The term “or” means any of thealternatives and any combination of the alternatives, including all ofthe alternatives, unless the alternatives are explicitly indicated asmutually exclusive. The phrase “at least one of” when combined with alist of items, means a single item from the list or any combination ofitems in the list. The phrase does not require all of the listed itemsunless explicitly so defined.

The above discussion is meant to be illustrative of the principles andvarious implementations of the present invention. Numerous variationsand modifications will become apparent to those skilled in the art oncethe above disclosure is fully appreciated. It is intended that thefollowing claims be interpreted to embrace all such variations andmodifications.

As used herein, the term “imprecise component” refers to a componentwith variance of more than ±50% around a nominal process corner, morethan 500 ppm/° C. variation in a temperature coefficient, more than 1000ppm/° C. nominal temperature coefficient, or any combination thereof.The imprecise components within an oscillator could allow anoscillator's accuracy to exceed ±1 percent across process, voltage andtemperature (PVT). An example of an imprecise component is apoly-resistor component that could have a nominal temperaturecoefficient of about 2,500 ppm/° C. which may vary from 1500 to 3300ppm/° C. over a fabrication process. Also used herein, the term “precisecomponent” refers to a component with variance of less than ±20 percentaround a nominal process corner, less than 200 ppm/° C. temperaturecoefficient, or both. The precise components within the oscillator couldallow the oscillator's accuracy to be equal to or less than ±1 percentacross PVT. Examples of precise components are a silicide poly-resistorcomponent that could have a temperature coefficient of about 100-200ppm/° C. or a zero temperature coefficient of resistance (ZTCR)component that could have a temperature coefficient of less than about50 ppm/° C.

Various example implementations are disclosed herein to reduce operationvariation for internal oscillators that do not contain precisioncomponents. In one or more implementations, the internal oscillators arerelaxation oscillators that include one or more comparators and at leastone current source that charge multiple capacitors. The comparators aresituated to compare the capacitor voltages to one or more referencevoltages in order to generate one or more periodic output signals. Tomanage the variability effects of including imprecise components, suchas a poly-resistor component, the internal oscillator is able to performtrimming operations for at least some of the imprecise components. As anexample, an internal oscillator may implement trimming operations withdigital-to-analog circuits (DAC), such as resistor DACs (RDACs), currentDACs (IDACs), and capacitor DACs (CDACs). The internal oscillatorincludes RDACs to trim and correct the resistance values of thepoly-resistor components. To further improve accuracy, an IDAC performsa temperature slope trim to adjust a first portion current of thecharging current supplied to the capacitors. Specifically, thetemperature slope trim can be based on correcting the resistor ratios ofthe poly-resistor components. After completing the temperature slopetrim, one or more CDACs may trim the charging capacitors to correct forany remaining variability or errors caused from process and/ortemperature variances.

FIG. 1 is a block diagram of an oscillator 100 in accordance withvarious implementations. In FIG. 1, the oscillator 100 is a non-crystalbased oscillator that includes a current source component 102, capacitorcomponents 104 and 106, comparator 108, and a Data (D) flip flop 110.Capacitor components 104 and 106 are capable of being coupled to thenon-inverting terminal of comparator 108 depending on whether switches112 and 114 are in a closed or open position. To generate the inputvoltage signal Vc at the non-inverting terminal of comparator 108, thecurrent source component 102 is also coupled to the non-inverting inputterminal of comparator 108 and charges capacitor components 104 or 106depending on the positions of switches 112 and 114. Capacitor components104 and 106 are also connected to switches 116 and 118 to allowcapacitor components 104 and 106 to discharge by connecting them to aground reference. The comparator 108 contains an inverting inputterminal that couples to a reference power circuit (not shown in FIG. 1)that supplies a reference voltage V_(REF). The output of the comparator108 is coupled to the clock (CK) input of the D flip flop 110 togenerate oscillating output signals labeled as ϕ and ϕ at the D flipflop's 110 output terminals. The output signal ϕ represents the positivevoltage half cycle of the oscillator's 100 output signal, and outputsignal ϕ represents the negative voltage half cycle of the oscillator's100 output signal. The output signal ϕ is connected to the D flip flop's110 data input terminal to form a feedback loop.

As shown in FIG. 1, oscillator 100 is a relaxation oscillator thatperforms current capacitor charging to generate capacitor voltage Vc andcompares the capacitor voltage Vc to a reference voltage V_(REF).Switches 112, 114, 116, and 118 are in open or closed states dependingon whether the oscillator 100 is generating a positive voltage halfcycle (e.g., a high output signal ϕ) or a negative half cycle (e.g., ahigh output signal ϕ)) as the oscillator's 100 output signal. When theoscillator 100 produces a positive voltage half cycle, switches 112 and118 are in a closed position while the switches 114 and 116 are in anopen position. During the positive voltage half cycle, the currentsource component 102 supplies current to capacitor component 104 viaswitch 112 to generate the capacitor voltage Vc. Capacitor voltage Vccontinues to increase to a point where capacitor voltage Vc meets and/orexceeds the voltage V_(REF). Closing switch 118 discharges any chargestored by capacitor component 106 by connecting the switch to the groundreference.

At the negative voltage half cycle, switches 114 and 116 are in theclosed position while switches 112 and 118 are in an open position. Thecurrent source component 102 provides current to capacitor component 106to generate the capacitor voltage Vc. Since capacitor component 106 wasdischarged during the positive voltage half cycle, the capacitorcomponent 106 initially provides a relatively low capacitor voltage Vc(e.g., 0 volts). Capacitor voltage Vc increase as the current sourcecomponent 102 charges capacitor component 106 via switch 114. Meanwhile,capacitor component 104 discharges any stored charge by closing switch116 and connecting the capacitor component 104 to the ground reference.

FIG. 2 illustrates a waveform diagram 200 that provide waveform graphs202, 204, and 206 that correspond to the operation of oscillator 100shown in FIG. 1. With reference to FIG. 1, waveform graph 202 plots thecapacitor voltage Vc at the non-inverting terminal of comparator 108 fora period of time; waveform graph 204 plots the output signal ϕassociated with positive voltage half cycle generated at the D flip flop110 for the same period of time; and waveform graph 206 plots the outputsignal ϕ associated with the negative voltage half cycle generated atthe D flip flop 110 for the same period of time. Each time the capacitorvoltage Vc reaches a voltage that meets and/or exceeds the referencevoltage V_(REF), the comparator pulses causing the D flip flop 110 tochange states.

As shown in waveform graph 202, the capacitor voltage Vc has a repeatingpattern that is similar to a sawtooth waveform or an asymmetric trianglewaveform. At sawtooth waves 208, switches 112 and 118 are in a closedposition to generate the positive voltage half cycle wave 216. Thevoltage increase for Vc at sawtooth waves 208 represents the capacitorcomponent 104 being charged by current source component 102. The slopeof the sawtooth waves 208, which represents the charge rate of capacitorcomponent 104, is dependent on the capacitor charging current Ic andcapacitance of capacitor component 104. As discussed above withreference to FIG. 1, while the current source component 102 charges thecapacitor component 104, capacitor component 106 is being discharged byclosing switch 118 and opening switch 114.

For sawtooth waves 210, switches 114 and 116 are in a closed position togenerate the negative voltage half cycle wave 220. The drop in voltageVc shown in the sawtooth wave 210 corresponds to transitioning switches114 and 116 to a closed position and switches 112 and 118 to an openposition. Previously, capacitor component 106 was discharged to arelatively low voltage (e.g., about 0 volts) causing the voltage Vc todrop at the beginning of sawtooth wave 210. Once the switches 112, 114,116, and 118 are in their updated position, the current source component102 then charges capacitor component 106; thereby, increasing voltage Vcsupplied to the comparator 108. Charging capacitor component 106 toincrease voltage Vc produces the negative voltage half cycle wave 220.The slope of the sawtooth waves 210, which represents the charge rate ofcapacitor component 106, is dependent on the capacitor charging currentIc and capacitance of capacitor component 106. As shown in waveformgraphs 202, 204, and 206, the sawtooth waves 208 and 210 continue torepeat in the same manner for the remaining time period to continuegenerating the output signals ϕ and ϕ.

Precision oscillators generally produce relatively high precision andstable output signals, such as having output signals set to a preciseand stable frequency. Referring back to FIG. 1, oscillator's 100frequency for output signals ϕ and ϕ can vary depending on thetemperature coefficient of the resistance value associated with thecurrent source component 102. In particular, the oscillator's 100 outputfrequency is based on the current source component's 102 suppliedcapacitor charge current Ic, the capacitance value of capacitorcomponents 104 and 106, and reference voltage V_(REF) supplied at theinverting terminal of the comparator 108. Equation 1 presented belowspecifically defines oscillator's 100 frequency value as:

$\begin{matrix}{F_{out} = \frac{I_{c}}{2{CV}_{REF}}} & (1)\end{matrix}$

In equation 1, F_(out) represents the output frequency of the oscillator100, which is associated with the output signals ϕ and ϕ; Ic representsthe capacitor charging current supplied by the current source component102; C represents the charging capacitance value that corresponds to oneof the charging capacitor components 104 and 106; and V_(REF) representsthe reference voltage supplied at the inverting terminal of thecomparator 108. The current source component 102 supplies the capacitorcharge current Ic, which is determined from the reference voltageV_(REF) and the resistance value of the current source component 102.Equation 2 presented below defines the capacitor charge current Ic as:

$\begin{matrix}{I_{c} = \frac{V_{REF}}{R}} & (2)\end{matrix}$

In equation 2, Ic represents the capacitor charging current supplied bythe current source component 102; V_(REF) represents the referencevoltage supplied at the inverting terminal of the comparator 108; and Rrepresents the resistance value of the current source component 102.From equation 2, equation 1 can be rewritten as shown in equation 3.

$\begin{matrix}{F_{out} = \frac{1}{2{RC}}} & (3)\end{matrix}$

As shown in equation 3, the oscillator's 100 output frequency is basedon the resistance value of the current source component 102 and thecapacitance value of the corresponding charging capacitor component 104or 106. Because of this dependency, the temperature coefficient of theresistance of the current source component 102 can affect theoscillator's 100 temperature coefficient of the output frequency.

In one or more implementations, the current source component 102 mayinclude one or more imprecise resistor components that cause thefrequency or time period for oscillator 100 to have a relatively largevariance. For example, the resistance value of current source component102 can be based on poly-resistor components that could potentiallycause about ±30 percent variance in resistance values depending on theoperation temperature and temperature coefficient at the process cornerthe device is in and about −60 percent to about +90 percent variancearound a nominal processor corner. The oscillator 100 can also sufferfrom variances from other components, such as capacitor components 104and 106 and comparator 108. As an example, capacitors 104 and/or 106 mayhave about a ±20 percent variance in capacitance and comparator 108 mayhave about a ±5 percent variance caused from comparator delay and othermargins. Because of process variations from one or more imprecisecomponents, the time period and frequency for oscillator 100 can overallvary from about −80 percent to about +200 percent. Stated another way,the oscillator 100 could have about a 13 times min-to-max variation inthe output frequency.

To reduce the impact of process and temperature variations associatedwith fabricating current source component 102, the current sourcecomponent 102 and capacitor components 104 and 106 may be configured astrimmable components. In one or more implementations, the current sourcecomponent 102 is able to perform a resistor trim that compensates forprocess variations for one or more imprecise resistor components (e.g.,poly-resistor component). As an example, the current source component102 may correct a first current portion of the capacitor chargingcurrent Ic defined as V_(REF)/R by utilizing a trimmable resistorcomponent, such as a RDAC. FIG. 5 provides more detail regarding theresistor trim. Additionally, the current source component 102 is able toperform a temperature slope trim to further tune the capacitor chargingcurrent Ic. The current source component 102 may utilize a trimmablecurrent component, such as an IDAC, to implement a second currentportion of the capacitor charging current defined as V_(PTAT)/R, whichis generated from a PTAT (proportional to absolute temperature) voltageto generate a target capacitor charging current Ic. FIG. 6 provides moredetail regarding the temperature slope trim. To further improveprecision for oscillator 100 (e.g., precise and stable frequency), thecapacitor components 104 and 106 may utilize capacitor trimmablecomponents that performs a capacitor trim to tune the capacitancevalues. For example, each capacitor component 104 and 106 may include aCDAC to adjust the capacitance values to obtain a target frequency foroscillator 100. FIG. 7 provides more detail for the capacitor trim.

FIG. 3 is a schematic diagram of another oscillator 300 in accordancewith various implementations. Similar to oscillator 100 shown in FIG. 1,oscillator 300 is a non-crystal-based, relaxation oscillator thatincludes a current source component 102 that provides charge current tocapacitor components 104 and 106. Stated another way, oscillator 300includes imprecise components that are susceptible to temperature andprocess variations. Oscillator 300 differs from oscillator 100 in thatoscillator 300 contains two comparators 302 and 304 and a set-reset (SR)latch 306. As shown in FIG. 3, the inverting terminals of comparators302 and 304 receive an input voltage of V_(REF). The non-invertingterminals of comparators 302 and 304 are each connected to one of thecapacitors 104 and 106 and the current source component 102 depending onwhether switches 112 and 114 are open and closed. As an example, thenon-inverting terminal of comparator 302 is connected to the currentsource component 102 and capacitor component 104 when switch 112 is in aclosed position and switch 114 is in an open position. In anotherexample, the non-inverting terminal of comparator 304 is connected tocurrent source component 102 and capacitor component 106 when switch 114is in a closed position and switch 112 is in an open position. Theoutput of comparator 302 connects to the reset input terminal of the SRlatch 306, and the output of comparator 304 connects to the set inputterminal of the SR latch 306. Based on the received inputs, the SR latch306 generates the output signals ϕ and ϕ.

FIG. 4 illustrates a waveform diagram 400 that include waveform graphs402, 404, and 406 that correspond to the operation of oscillator 300.Waveform graph 402 plots the output signal ϕ associated with a positivevoltage half cycle generated at the SR latch's 306 Q output terminalover a period of time; waveform graph 404 plots the output signal ϕassociated with the negative voltage half cycle generated at the SRlatch's 306 Q output terminal; waveform graph 406 plots the capacitorvoltage X (shown in FIG. 3) at the non-inverting terminal of comparator302; and waveform 408 pertains to the capacitor voltage Y (shown in FIG.3) at the non-inverting terminal of comparator 304.

As shown in waveform graphs 402 and 404, the capacitor voltage X startswith a relatively low voltage (e.g., 0 volts) and capacitor voltage Yproduces a sawtooth wave 416. When the capacitor voltage Y meets and/orexceeds the reference voltage V_(REF), comparator 304 pulses and causesthe SR latch 306 to change states. Subsequently, capacitor voltage Xtransitions to a sawtooth wave 414 and capacitor voltage Y transitionsto a relatively low voltage. Similar to capacitor voltage Y, whencapacitor voltage X meets and/or exceeds the reference voltage V_(REF),comparator 302 pulses and causes the SR latch 306 to change states. Atthe sawtooth wave 414, switches 112 and 118 are in a closed position andswitches 114 and 116 are in an open position to generate the positivevoltage half cycle wave. At the sawtooth waves 416, switches 112 and 118are in an open position and switches 114 and 116 are in a closedposition to generate the negative voltage half cycle wave.

Referring back to FIG. 3, to reduce the impact of process andtemperature variations associated with fabricating current sourcecomponent 102, oscillator 300 is able to perform trim operations at thecurrent source component 102 and capacitor components 104 and 106.Rather than utilizing a single magnitude trim circuitry to trim thefrequency of oscillator 300 within a ±1 percent accuracy, the oscillator300 may implement a trim circuit that trims multiple components withinthe oscillator 300. The current source component 102 includes trimmableresistor components that can be trimmed to compensate for the processvariation for one or more imprecise resistor components (e.g.,poly-resistor component). Additionally, the current source component 102includes a trimmable current component that performs a temperature slopetrim to further tune the capacitor charging current Ic by compensatingfor temperature coefficient variation. To improve precision andstability of oscillator 300, the capacitor components 104 and 106 mayalso include trimmable capacitor components that tune the capacitancevalues. The different trim operations are referenced and discussed inmore detail in FIGS. 5-7.

Although FIGS. 1 and 3 illustrate specific implementations ofoscillators 100 and 300, the disclosed trimming operations are notlimited to the specific implementation illustrated in FIGS. 1 and 3. Forinstance, switches 112, 114, 116, and 18 within FIGS. 1 and 3 may beimplemented using a variety of different types of electrical switches,such as field-effect transistors (FETs). Examples of FETs include, butare not limited to enhanced n-channel metal-oxide semiconductorfield-effect (NMOS) transistors, other types of NMOS transistors, an-type junction gate field-effect transistor (NJFET), and a bipolarjunction transistor (BJT) (e.g., NPN transistors). Additionally, ratherthan utilizing a D flip flop or a SR latch, a relaxation oscillator mayemploy another flip flop type component, latch type component, or someother type of electrical circuit configured to store state informationand generate multiple stable states. The use and discussion of FIGS. 1and 3 are only an example to facilitate ease of description andexplanation.

FIG. 5 is a schematic diagram of an implementation of a trim-capablecurrent source component 500 configured to perform a resistance valuetrim and temperature slope trim. As previously discussed, thetrim-capable current source component 500 may be fabricated withfabrication and/or process technology that create imprecise componentsthat have relatively large temperature coefficients, which in turnvaries the capacitor charging current Ic based on temperature. Tocompensate for the relatively large process variations and temperaturecoefficients, the trim-capable current source component 500 is able toperform a resistance value trim using the RDAC components 504 and 510and a temperature slope trim using a trimmable current component 520that is part of the bias circuit 519. Temperature slope trimming with atrimmable current component 520, such as an IDAC component thatcompensates for temperature coefficient variation will be discussed inmore detail in FIG. 6. The RDAC components 504 and 510 may be trimmedtogether to obtain a target resistance value to account for processvariations.

In FIG. 5, the RDAC component 504 has one end that connects to aninverting terminal of amplifier 502 and an opposite end that connects tothe non-inverting terminal of amplifier 506. The end of the RDACcomponent 504 that connects to an inverting terminal of amplifier 502also connects to the source node of transistor 522. The RDAC component510 has one end that connects to both the inverting terminal ofamplifier 508 and the source node of transistor 525. The opposite end ofRDAC component 510 connects to a ground reference. A constant voltageV_(CON) represents the voltage supplied to the inverting terminal ofamplifier 508 and also the voltage drop across RDAC component 510.

In one or more implementations, the RDAC components 504 and 510 areimprecise components (e.g., poly-resistor components) that haverelatively large process variation. The RDAC components 504 and 510 areconfigured to adjust their resistance values in order to offset therelatively large process variation associated with each component. Inone or more implementations, the RDAC components 504 and 510 may bepoly-resistor components that have a process variation that ranges from−60% to +90%. As shown in FIG. 5, since a first current portion ICON ofthe charging current Ic is equal to V_(CON)/R_(RDAC), any resistancevariance in the RDAC component 510 affects the generated first currentportion ICON. To compensate for varying resistance values caused fromprocess variation, the RDAC component 510 corrects the first currentportion ICON by trimming to a resistance value R_(RDAC). Thetrim-capable current source component 500 may trim the RDAC components504 and 510 at a designated temperature (e.g., room temperature) to aresistance value R_(RDAC) that is within a desired accuracy range (e.g.,within 5-6% accuracy) of a target resistance value. As an example,because of process variation, the RDAC component 510 may have aresistance value at room temperature of 1.20 kilo-ohm when the targetresistance value is set to 1.00 kilo-ohm. The RDAC component 510performs a resistor trim to lower the actual resistance value R_(RDAC)to less than or equal to 1.05 kilo-ohm. FIG. 5 also illustrates that theRDAC component 504 performs a similar resistor trim to have a resistancevalue that matches the resistance value R_(RDAC).

To determine whether the resistance values for RDAC components 504 and510 have drifted away from the target resistance value R_(RDAC), thetrim-capable current source component 500 includes a trim system 514 anda resistor trim control 512. The trim system 514 includes a currentmeasurement component 516 (e.g., ammeter) that measures the firstcurrent portion ICON at a designated temperature (e.g., roomtemperature). To measure the first current portion ICON, the resistortrim control 512 sets switch 518 to a close state to allow current toflow to the current measurement component 516. When switch 518 is in theclose state, transistor 532 mirrors the first portion current ICON tothe test pin 534 of the trim system 514 for measuring. Based on themeasured current, which mirrors the first portion current ICON, the trimsystem 514 determines the offset between the measured resistance valuefor RDAC component 510 and the target resistance value. The trim system514 then translates the resistance offset into resistor trim bits tosupply to the resistor trim control 512. The resistor trim control 512utilizes the resistor trim bits to adjust the resistance value for theRDAC component 510 to be closer the target resistance value. Theresistor trim control 512 also adjusts the resistance value of the RDACcomponent 504 to the target resistance value based on the measuredcurrent.

In one or more implementations, to adjust the resistance values, each ofthe RDAC components 504 and 510 may include multiple switches 528 thatset which resistors 526 are connected in series to a fixed resistor 530.In FIG. 5, each of the resistors 526 are set to have the same resistancevalue of AR that increases the resistance from a fixed resistance valueR_(FIX). In other implementations, each resistor 526 can be set todifferent resistance values, such as ΔR, Δ2R, and/or Δ4R, to increasethe resistance of the RDAC components 504 and 510. Based on the resistortrim bits, the resistor trim control 512 sets switches 528 to open orclose positions. For instance, resistor trim control 512 may receiveresistor trim bits indicating that only the fixed resistor 530 withresistance of value of R_(FIX) should be connected to the invertingterminal of amplifier 508. In another instance, the resistor trimcontrol 512 may receive resistor trim bits indicating that two of theresistors 526 should be connected in series to the fixed resistor 530 tobe closer to the target resistance value.

FIG. 6 is a schematic diagram of an implementation of trim-capablecurrent source 600 configured to perform a temperature slope trim tocompensate for temperature variation relating to generating thecapacitor charging current Ic. As previously referenced in equation 1,the frequency of an oscillator depends on the capacitor charging currentIc. Increasing the capacitor charging current Ic increases the frequencyof the oscillator. In particular, with reference to FIG. 2, increasingthe capacitor charging current Ic increases the slope of the sawtoothwaves 208 and 210, which allows the capacitor voltage Vc to equal and/orexceed the reference voltage V_(REF) in a shorter time frame.Conversely, decreasing the capacitor charging current Ic decreases thefrequency of the oscillator, which decreases the slope of the sawtoothwaves 208 and 210. Once the trim-capable current source 600 trims theRDAC components 504 and 510 to compensate for the components' processvariation, the trim-capable current source 600 obtains a correct ratioof V_(CON) and V_(PTAT) to reduce temperature variation and produce atarget capacitor charging current Ic.

The capacitor charging current Ic is derived from a combination of aproportioned current ICON and current I_(PTAT). Equation 4 presentedbelow defines the capacitor charging current Ic.

$\begin{matrix}{I_{c} = \frac{{\beta \; V_{CON}} + V_{PTAT}}{R_{RDAC}}} & (4)\end{matrix}$

In equation 4, β represents a slope trim coefficient; Ic represents thecapacitor charging current supplied by the current source component 102;V_(CON) represents the voltage received at amplifier 508 and the voltagedrop across RDAC component 510; V_(PTAT) represents the voltage dropacross RDAC component 504; and R_(RDAC) represents the trimmedresistance value for RDAC components 504 and 510. With regards to βwithin equation 4, as the slope trim code changes, β will also change inorder to manipulate the overall slope of frequency. Based on equation 4,the capacitor charging current Ic is the sum of the current I_(PTAT) andthe proportioned current ICON.

From equation 4, equation 1 can be rewritten as shown in equations 5 and6 to determine the frequency of the oscillator.

$\begin{matrix}{F_{out} = {\frac{{\beta \; V_{CON}} + V_{PTAT}}{V_{REF}}*\left( \frac{1}{2\; R_{RDAC}C} \right)}} & {{~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~}(5)} \\{= {\left( \frac{\beta \; V_{CON}}{V_{REF}} \right)*\left( {1 + \frac{V_{PTAT}}{\beta \; V_{CON}}} \right)*\left( \frac{1}{2\; R_{RDAC}C} \right)}} & {(6)}\end{matrix}$

In equations 5 and 6, F_(out) represents the output frequency of theoscillator; Ic represents the capacitor charging current supplied by thecurrent source component 102; V_(CON) represents the constant voltagereceived at amplifier 508 and the voltage drop across RDAC component510; V_(PTAT) represents the voltage drop across RDAC component 504;R_(RDAC) represents the trimmed resistance value for RDAC components 504and 510; V_(REF) represents the reference voltage supplied to thecomparators; and C represents the capacitance value of the correspondingcharging capacitor components (e.g., one of the capacitor components 104and 106 in FIGS. 1 and 3). Based on equation 6, the ratio of V_(CON) andV_(PTAT) affects the output frequency of the oscillator.

The PTAT current may be readily available from bandgap and/or can bederived from PTAT voltages from bandgap. In FIG. 6, a single bandgapcircuit is used to generate the constant voltage V_(CON) and the twovoltage V_(BG) and V_(BE). Recall that the constant voltage V_(CON) issupplied to the non-inverting terminal of amplifier 508. Voltage V_(BG)is supplied to the non-inverting terminal of amplifier 502, and voltageV_(BE) is supplied to the inverting terminal of amplifier 506. VoltagesV_(CON) and V_(BG) have a constant voltage while voltage V_(BE) hasnegative slope with temperature. Voltage V_(PTAT) is equal to thedifference in voltage V_(BG) and V_(BE). The accuracy of the bandgapvoltage depends on the resistor ratio instead of absolute resistancevalues, and hence the resistors used in bandgap design can be imprecisecomponents (e.g. the same type of resistor used in R_(RDAC) components504 and 510) rather than requiring a precision circuit with precisionresistors. As shown in FIG. 6, a complementary to absolute temperaturethat reduces proportionally with temperature (CTAT) circuit is not usedto generate the target capacitor charging current Ic.

Trim-capable current source 600 utilizes trimmable current component 520to perform temperature slope trim that reduces the temperaturecoefficient of frequency for an oscillator. As an example, trimmablecurrent component 520 may be an IDAC that corrects the ratio of voltageV_(CON) and voltage V_(PTAT) to obtain a target capacitor chargingcurrent Ic that contributes in achieving a precise and stable oscillatorfrequency. In FIG. 6, the trimmable current component 520 is configuredto modify the current ICON through digital control. In implementationswhere the trimmable current codes is for an IDAC, the trim-capablecurrent source 600 may adjust the trimmable current codes (e.g., IDACcodes) to tune the IDAC in order to increase or decrease the proportioncurrent ICON. Increasing the proportion of current ICON reduces theoverall temperature slope of the oscillator's frequency. Conversely,decreasing the proportion of current ICON would increase the overalltemperature slope of the oscillator's frequency. As show in FIG. 6, toincrease or decrease the proportion of current ICON, trimmable currentcomponent 520 acts as current mirrors that enable one or more of thetransistors 604 to obtain the desired first portion current ICON.Switches 606 are used to enable the transistors 604 within trimmablecurrent component 520 by setting the switches to an open state or aclose state based on the trimmable current codes.

FIG. 7 illustrates a trim-capable current source 700 utilizes trimmablecurrent component 720 to perform temperature slope trim that reduces thetemperature coefficient of frequency for an oscillator. Similar to FIG.6, trimmable current component 720 may be an IDAC that corrects theratio of voltage V_(CON) and voltage V_(PTAT) to obtain a targetcapacitor charging current Ic that contributes in achieving a preciseand stable oscillator frequency. In contrast to FIG. 6, the trimmablecurrent component 720 is configured to modify the current I_(PTAT)through digital control. In implementations where the trimmable currentcodes is an IDAC, the trim-capable current source 700 may adjust thetrimmable current codes (e.g., IDAC codes) to tune the IDAC in order toincrease or decrease the proportion current I_(PTAT). Increasing theproportion of current I_(PTAT) increases the overall temperature slopeof the oscillator's frequency. Conversely, decreasing the proportion ofcurrent I_(PTAT) would decrease the overall temperature slope of theoscillator's frequency. As show in FIG. 7, to increase or decrease theproportion of current I_(PTAT), trimmable current component 720 acts ascurrent mirrors that enable one or more of the transistors 704 to obtainthe desired second portion current I_(PTAT). Switches 606 are used toenable the transistors 704 within trimmable current component 720 bysetting the switches to an open state or a close state based on thetrimmable current codes.

FIG. 8 is a schematic diagram of an embodiment of trimmable capacitorcomponents 804 and 806. Using FIG. 1 as an example, the trimmablecapacitor components 804 and 806 correspond to or are part of thecapacitor components 104 and 106, respectively. After performing thetemperature slope trim, remaining magnitude error for achieving a targetoutput frequency for an oscillator may be corrected with the trimmablecapacitor components 804 and 806. Recall that in FIG. 6, thetrim-capable current source 600 performs a temperature slope trim. Whenimplementing a temperature slope trim to correct the temperature slopeof the oscillator's frequency, the increase in the first proportioncurrent ICON tends to increase the frequency magnitude of the oscillatorand decreasing the first proportion current ICON tends to decrease thefrequency magnitude. The increase and decreasing of the frequencymagnitude could cause a residual magnitude error of frequency that maybe corrected using the trimmable capacitor components 804 and 806.

In one or more implementations, each trimmable capacitor components 804and 806 is a CDAC that trims the charging capacitor. Using FIG. 8 as anexample, the trimmable capacitor components 804 and 806 include two 10bit split-CDAC arrays, where two 10 bit split-CDAC arrays are identicalto each other and utilize the same 10 bit trim code to regulate thecapacitance charging time. Both trimmable capacitor components 804 and806 are configured to include a capacitor 808 with a fixed capacitanceC_(fix) that connects in parallel with capacitor 830 that has acapacitance of C. Capacitor 832 is connected to capacitors 808 and 830in series and has a capacitance of 32C/31. The two 10 bit split-CDACarrays include capacitors 810, 812, 814, 816, 818, 820, 822, 824, 826,and 828 that have capacitance 16C, 8C, 4C, 2C, C, 16C, 8C, 4C, 2C, andC, respectively. Other embodiments of the 10 bit split-CDAC array may beconfigured to assign different capacitance values for capacitors 810,812, 814, 816, 818, 820, 822, 824, 826, and 828.

FIG. 8 illustrates that the trimmable capacitor components 804 and 806utilize a 10 bit trim code with bits b_(9:0) to perform capacitorswitching of capacitors 810, 812, 814, 816, 818, 820, 822, 824, 826, and828. As shown in FIG. 8, capacitors 810, 812, 814, and 816 are part ofthe CDAC array that corresponds to the most significant bits of the 10bit trim code and capacitors 818, 820, 822, 824, and 826 are part of theCDAC array that correspond to the least significant bits of the 10 bittrim code. Specifically, capacitors 810, 812, 814, and 816 representstrim bits b_(9:5) and capacitors 818, 820, 822, 824, and 826 representstrim bits b_(4:0) of the 10 bit trim code. The bottom plates ofcapacitors 810, 812, 814, 816, 818, 820, 822, 824, 826, and 828 mayswitch from ground to reference voltage V_(REF) depending on whethertheir corresponding trim bits b_(9:0) are set to a designated logicalvalue (e.g., logic value one). As an example, if bit b₉ of the 10 bittrim code is set to a logic value of one, the bottom plate of capacitor810 is connected to the reference voltage V_(REF). Alternatively, if bitb₉ of the 10 bit trim code is set to a logic value of zero, the bottomplate of capacitor 810 is connected to a ground reference.

In one half cycle (e.g., the positive voltage half cycle), capacitors810, 812, 814, 816, 818, 820, 822, 824, 826, and 828 for one of the CDACarrays will be grounded and discharged, while in the other CDAC array,the bottom plates of capacitors 810, 812, 814, 816, 818, 820, 822, 824,826, and 828 switch from ground (e.g., 0 volts) to reference voltageV_(REF). In the next half cycle (e.g., the negative voltage half cycle)the roles for the CDAC arrays will be reversed. By having the bottomplates of capacitors 810, 812, 814, 816, 818, 820, 822, 824, 826, and828 switch from ground (e.g., 0 volts) to reference voltage V_(REF),nodes A and B in FIG. 8 are pre-charged to fraction of V_(REF).Pre-charging to a fraction of V_(REF) in turn controls charging time forthe trimmable capacitor components 804 and 806, the time period of theoscillator, and the frequency of the oscillator. Equation 7 presentedbelow defines the capacitor charging current Ic based on a 10 bit trimcode.

$\begin{matrix}{T = {\frac{2\; V_{REF}}{I_{C}}*\left\lbrack {C_{fix} + {32\; {C\left( {1 - \frac{1{\sum\limits_{0}^{9}\; {b_{m}2^{m}}}}{1024}} \right)}}} \right\rbrack}} & (7)\end{matrix}$

In equation 7, T represents the time period for the oscillator; Icrepresents the capacitor charging current supplied by the current sourcecomponent; V_(REF) represents the reference voltage supplied to thecomparators; C_(fix) represents the capacitance value of capacitor 808within the CDAC; C represents the capacitance value of capacitors 818and 828; and b_(m) represents the trim bit of the 10 bit trim code.Equation 7 can be modified accordingly based on the number of trim bits.

Although FIG. 8 illustrates utilizing a CDAC array for a 10 bit trimcode, other embodiments could use other trim code lengths depending onthe trim range. As an example, rather using one or more 10 bitsplit-CDAC array, the oscillator may include one or more 8 bit CDACarrays that provide for lower trimming resolution. Within the 8 bit CDACarray, bits b_(7:0) may be associated with varying capacitance values.In one implementation, bit b₀ to b₇ could have capacitance values of C,2C, 4C, 8C, 16C, 32C, 64C, and 128C, respectively. An oscillator may usethe 8 bit CDAC rather than a 10 bit split-CDAC array depending on theprocess technology of the trimmable capacitor components 804 and 806and/or the amount of comparator delay.

FIG. 9 is a flow chart of an implementation of a method 900 to trimmultiple components within a precision oscillator to stabilize and reacha target frequency. Using FIGS. 1 and 3 as examples, method 900 can beimplemented for the oscillators 100 and 300. Method 900 may also beapplication to other types of relaxation oscillators that utilize acurrent source to charge capacitors and compares the capacitor voltagesto a reference voltage. Method 900 starts at block 902 to trim one ormore trimmable resistor components with a current source component tocompensate for process variations of the trimmable resistor components.With reference to FIG. 5, method 900 performs a trim to compensate forvarying resistance values originating from process variations using RDACcomponents 504 and 510. Method 900 may determine whether the resistancevalues for RDAC components 504 and 510 are outside a target range (e.g.,within 5-6% of the target resistance value). If so, method 900translates the resistance offset into resistor trim bits to adjust theresistance value for RDAC components 504 and 510 to be closer the targetresistance value.

Method 900 may then move to block 904 and perform a temperature slopetrim within the current source component to compensate for the frequencytemperature coefficient of the oscillator. Recall in FIG. 6, thatcapacitor charging current Ic affects the frequency of the oscillator.Method 900 performs the temperature slope trim to obtain a correct ratiobetween the first current portion ICON and second current portionI_(PTAT). In other words, method 900 may increase or decrease theproportion of constant current ICON to reach target frequency output forthe oscillator. Method 900 may then continue to block 906 and trimresidual magnitude error within one or more capacitor components. Asdiscussed above with reference to FIG. 9, method 900 may implement block906 using a CDAC that trims the charging capacitor. In one or moreimplementations, method 900 may trim the capacitance according to a 10bit trim code that switch the bottom plates of capacitors within theCDAC array from ground (e.g., 0 volts) to reference voltage V_(REF).Other embodiments may utilize other trim code resolution, such as a 9bit trim code.

At least one embodiment is disclosed and variations, combinations,and/or modifications of the embodiment(s) and/or features of theembodiment(s) made by a person having ordinary skill in the art arewithin the scope of the disclosure. Alternative embodiments that resultfrom combining, integrating, and/or omitting features of theembodiment(s) are also within the scope of the disclosure. Wherenumerical ranges or limitations are expressly stated, such expressranges or limitations may be understood to include iterative ranges orlimitations of like magnitude falling within the expressly stated rangesor limitations (e.g., from about 1 to about 10 includes, 2, 3, 4, etc.;greater than 0.10 includes 0.11, 0.12, 0.13, etc.). The use of the term“about” means±10% of the subsequent number, unless otherwise stated.

While several embodiments have been provided in the present disclosure,it should be understood that the disclosed systems and methods might beembodied in many other specific forms without departing from the spiritor scope of the present disclosure. The present examples are to beconsidered as illustrative and not restrictive, and the intention is notto be limited to the details given herein. For example, the variouselements or components may be combined or integrated in another systemor certain features may be omitted, or not implemented.

In addition, techniques, systems, subsystems, and methods described andillustrated in the various embodiments as discrete or separate may becombined or integrated with other systems, modules, techniques, ormethods without departing from the scope of the present disclosure.Other items shown or discussed as coupled or directly coupled orcommunicating with each other may be indirectly coupled or communicatingthrough some interface, device, or intermediate component whetherelectrically, mechanically, or otherwise.

What is claimed is:
 1. An oscillator comprising: a trim-capable currentsource, wherein the trim-capable current source comprises a trimmableresistor and a trimmable current component; a comparator comprising afirst input terminal that couples to the trim-capable current source anda second input terminal that couples to a reference voltage source; aswitch coupled to the first input terminal and the trim-capable currentsource; and a trim-capable capacitor coupled to the switch, wherein theswitch is coupled between the trim-capable capacitor and thetrim-capable current source.
 2. The oscillator of claim 1, furthercomprising: a second switch coupled to the first input terminal and thetrim-capable current source; and a second trim-capable capacitor coupledto the first input terminal, where the second switch is positionedbetween the trim-capable capacitor and the trim-capable current source.3. The oscillator of claim 1, further comprising a flip flop component,wherein an output terminal of the comparator is coupled to an input ofthe flip flop component.
 4. The oscillator of claim 1, furthercomprising: a second comparator comprising a third input terminal and afourth input terminal, wherein the fourth input terminal is coupled tothe reference voltage source; and a second switch coupled between thetrim-capable current source and the third input terminal of the secondcomparator.
 5. The oscillator of claim 4, further comprising a secondtrim-capable capacitor coupled to the third input terminal.
 6. Theoscillator of claim 4, further comprising a flip flop, wherein an outputterminal of the comparator is coupled to an input of the flip flop andan output terminal of the second comparator is coupled to a second inputof the flip flop.
 7. The oscillator of claim 1, wherein the trimmableresistor includes a resistor digital-to-analog circuit (RDAC) and thetrimmable current component includes a current digital-to-analog circuit(IDAC).
 8. The oscillator of claim 1, wherein the trim-capable currentsource comprises a second trimmable resistor.
 9. The oscillator of claim8, wherein the trimmable resistor and the second trimmable resistor areimprecise resistors.
 10. The oscillator of claim 1, wherein thetrim-capable capacitor is a capacitor digital-to-analog circuit (CDAC).11. An oscillator comprising: a trim-capable current source configuredto: trim one or more resistance values that affects a first portioncurrent of a capacitor charging current generated from the trim-capablecurrent source; and trim the first portion current to adjust aproportion of the first portion current in the capacitor chargingcurrent generated from the trim-capable current source; a comparatorcomprising a first input terminal that couples to the trim-capablecurrent source and a second input terminal that couples to a referencevoltage source; and a trim-capable capacitor coupled to the first inputterminal, wherein the trim-capable capacitor is configured to performcapacitor switching that controls a charge time of the trim-capablecapacitor.
 12. The oscillator of claim 11, wherein the trim-capablecurrent source comprises a resistor digital-to-analog circuit (RDAC) anda current digital-to-analog circuit (IDAC).
 13. The oscillator of claim12, wherein the RDAC includes a plurality of poly-resistors.
 14. Theoscillator of claim 11, wherein the trim-capable current source isconfigured to generate a proportional to a second portion current of thecapacitor charging current that combines with the first portion currentto form the capacitor charging current.
 15. The oscillator of claim 11,wherein the trim-capable capacitor is a capacitor digital-to-analogcircuit (CDAC).
 16. The oscillator of claim 15, wherein the CDACincludes a 10 bit split-CDAC array.
 17. A method comprising: trimming,for a relaxation oscillator, one or more resistance values that affectsa first portion current of a capacitor charging current generated from atrim-capable current source located within the relaxation oscillator;trimming, for the relaxation oscillator, the first portion current toadjust a proportion of the first portion current in the capacitorcharging current generated from the trim-capable current source, whereinthe capacitor charging current is a combination of the first portioncurrent and a second portion current; and trimming, for the relaxationoscillator, a trim-capable capacitor to perform capacitor switching thatcontrols a charge time of the trim-capable capacitor.
 18. The method ofclaim 17, wherein the trim-capable current source includes a resistordigital-to-analog circuit (RDAC) to trim the one or more resistancevalues and a current digital-to-analog circuit (IDAC) to trim the firstportion current that adjusts the proportion of the first portion currentin the capacitor charging current.
 19. The method of claim 17, whereinthe trim-capable capacitor is a capacitor digital-to-analog circuit(CDAC).
 20. The method of claim 19, wherein the CDAC includes a 10 bitsplit-CDAC array.